Early models of the solar system described the Earth as stationary at the center with celestial objects revolving around it in circular orbits. Ptolemy developed an influential geocentric model that explained observations using epicycles but was inaccurate. Copernicus proposed a heliocentric model placing the Sun at the center. Kepler analyzed Brahe's precise observations of Mars to deduce its elliptical orbit and formulated his three laws of planetary motion, establishing the foundations of modern astronomy.